Perhaps the most widely used basic concept in oil shale retorting involves countercurrently contacting a stream of crushed oil shale with a stream of preheated eduction gas. In this manner, a temperature gradient is set up in the moving shale bed, including a hot eduction zone near the gas inlet to the retort, and an oil condensation, shale preheating zone near the shale inlet end of the retort. In the solids downflow mode of operation wherein the shale is moved by gravity, a persistent problem has been that of shale agglomeration. At temperatures in the range of about 750.degree.-850.degree. F., the shale particles become somewhat plastic in nature and tend to form agglomerates which seriously impede gas flow, and may in some cases actually cause bridging of the retort with total stoppage of solids flow. To the best of our knowledge, this problem of agglomeration in solids-downflow retorting has never heretofore been solved in any practical manner. Another major problem in this type of retorting has been that of preventing condensed shale oil in the cool, upper condensation zone from refluxing downwardly into the hot eduction zone, with resultant excessive cracking.
Considerable progress was made in solving the foregoing problems by resorting to the solids-upflow, gas-downflow retorting technique such as that described in U.S. Pat. No. 3,361,644. In this process oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving shale bed continuously exchanges heat with a downflowing high specific heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 950.degree.-1200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the shale. In the lower sections of the retort the oil shale is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed product vapors. Most of the heavier hydrocarbons condense in the lower portion of the retort, and are continually swept away from the hot pyrolysis zone and are collected at the bottom of the retort as product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain more product oil. The remaining gases are then utilized as high BTU product gas, recycle gas as above described, and as fuel gas to preheat the recycle gas up to the above specified temperatures.
While the solids upflow technique completely avoids the refluxing and overcracking problem encountered in solids downflow processes, it does not in all cases completely avoid the solids agglomeration problem. Although there is no danger of the retort becoming bridged by agglomerated shale, sufficient agglomeration does in some cases occur to impede uniform gas flow, with resultant excessive pressure drops, reduced oil yields and diminished throughput rates. Here again, the critical region of agglomeration is the 750.degree.-850.degree. F. zone in the retort. Although the rock pressure bearing on this zone is less than that normally prevailing in solids downflow retorting, it has been found that under some conditions the rock pressure is sufficient to bring about some agglomeration in that zone. (It will be understood that at temperatures below about 750.degree., the shale particles have not yet reached a plastic stage, and that after passing through the 750.degree.-850.degree. F. zone sufficient eduction has occurred to again render the shale particles non-plastic.)
We have now discovered however that there is a relationship which exists between the oil assay of the fresh shale and the rock pressure which will bring about agglomeration in the 750.degree.-850.degree. F. zone. Shales having a high oil assay, above about 35 gallons per ton, will agglomerate and impede gas flow at solids pressures above about 3 psi bearing upon the 750.degree.-850.degree. F. zone. Shales of lower assay can withstand considerably higher solids pressures without agglomeration. We have found that the level of the 750.degree.-850.degree. F. temperature interval in the retort can be controlled in response to oil assay of the raw shale so as to render the rock pressure bearing upon that interval insufficient to bring about significant agglomeration, in either solids-upflow or -downflow retorting.
The overall objective is to provide sufficient total heat input into the retort to obtain the desired solids temperature at the solids outlet end of the retort (900.degree. F. or higher) without heating the solids to the 750.degree.-850.degree. F. range at a level in the retort where the rock pressure is high enough to cause agglomeration. This objective is achieved by selecting the right combination of recycle gas rate and temperature. At the same total heat input, the position of the critical 750.degree.-850.degree. F. zone can be raised or lowered in the retort by proper correlation of recycle gas rate and temperature. The combination of high temperatures and low recycle gas rates will elevate the critical zone sufficiently to avoid agglomerating pressures for any oil shale of up to about 60 gallons per ton Fischer assay. However, it is usually undesirable to maintain high recycle gas temperatures unless a high assay shale is being retorted, for preheating the recycle gas to above about 1050.degree. F. tends to cause thermal cracking of hydrocarbons in the recycle gas, and more rapid coking of the recycle gas heater. When shales of lower oil assay are being retorted, it is therefore preferable to reduce the recycle gas temperature and increase the flow rate thereof. This lowers the level of the 750.degree.-850.degree. F. zone. However the recycle gas rate should not be too high or the shale will reach the critical 750.degree.-850.degree. F. temperature range at a level where solids pressures are too high and agglomeration can occur.
We have now found that high oil yields and high throughput rates can be obtained without agglomeration in a solids upflow retort when feeding shales of 30-35 gallons per ton Fischer assay by adjusting the recycle gas rate and recycle gas inlet temperature to obtain a temperature profile which approaches a uniform slope when temperature is plotted against bed height. When richer shales are fed to the retort, to the point where agglomeration and plugging begin to occur, such agglomeration and plugging can be avoided by decreasing the recycle gas flow rate and increasing the gas inlet temperature so as to obtain a higher rate of solids temperature increase per unit change in bed height near the top of the retort as compared to the rate of temperature increase per unit change in bed height in the bottom half of the retort.
From the foregoing, it will be apparent that there is a critical intermediate temperature interval of about 750.degree.-850.degree. F. in the shale bed. Further, the horizontal level of this intermediate temperature zone can be shifted upwardly or downwardly in the shale bed by suitably correlating eduction gas temperature and flow rate. Also, the middle of said intermediate temperature zone, i.e., the 800.degree. F. level, should not be below a critical level F. which shale agglomeration at any point within said zone becomes significant, said critical level being directly related to the oil assay of the shale being fed to the retort. Resorting to this relationship, said critical level can be defined as that level at which the rock pressure bearing thereon in pounds per square inch is about (120,000/(F.A.).sup. 3 + 0.5, where F. A. is the Fischer assay of said shale, in gallons per ton. Preferably however, the 800.degree. F. level of said intermediate temperature zone is no lower in the bed than the level at which the rock pressure is (100,000/(F.A.).sup.3 + 0.5 pounds per square inch. However, in order to minimize thermal cracking of hydrocarbons in the recycle gas preheater, as well as over-cracking in the upper high-temperature portion of the shale bed, the top of said intermediate temperature zone, i.e. the 850.degree. F. level, should preferably be at least about 1 foot below the top of the shale bed in a solids upflow retort. (It should be noted that the foregoing formulations based on rock pressure cannot be accurately and conveniently expressed in terms of bed depth; the former is not a simple linear function of the latter.)
One aspect of the invention contemplates the retorting of shale feeds which may vary substantially, e.g. by at least 5 gallons per ton, in oil assay from time to time. In such cases, the intermediate temperature zone will be maintained at a relatively low level in the shale bed when the shale feed assays relatively lean in oil, and at a higher level when the feed assays relatively rich in oil, the lower level and the higher level each preferably being at least about one foot below the top of the bed, but sufficiently high therein to substantially prevent agglomeration of shale particles.